But there are signs of slippage heading into Tuesday's nominating contest in Indiana, a conservative Midwestern U.S. state that has voted Republican in nine of the last 10 presidential elections.
Trump, 69, has taken stances on Planned Parenthood family clinics and gay and transgender rights that raise Christian conservative concerns, including in such states as Indiana where they make up a high proportion of voters.
A new NBC/Wall Street Journal/Marist opinion poll shows Trump with a wide lead in Indiana, 49 percent, to 34 percent for his nearest rival, U.S. Senator Ted Cruz of Texas, and 13 percent for Ohio Governor John Kasich.
RELATED: Here are the latest power rankings on who's mostly likely to be the next president
April 26 Presidential Power Rankings (BI)
Indiana puts Donald Trump's evangelical support to test
5. John Kasich, Republican, Ohio governor
Kasich picked up momentum last month with a win in his home state's primary.
But the road before, and since, has been virtually empty for the Ohio governor. He has no chance to accumulate enough delegates to clinch the nomination before the convention, so he's banking that he can win a floor fight.
Those who talk up Kasich say he is a successful governor of a swing state with a record to point to and clear bipartisan appeal. He also has abundant experience from nearly two decades in Congress, including foreign-policy areas and his time as chair of the US House budget committee.
But that same bipartisan brand has hurt Kasich with the GOP base. He is to the left of most GOP candidates on immigration reform, and he expanded the federal Medicaid program under the Affordable Care Act — two issues that could doom him with hard-line conservatives.
National polling average among Republican voters: 22.3% (3rd)
Average in Pennsylvania, Maryland, Connecticut, and Indiana polls: 23.9% (3rd)
Last month: 5
(Photo via REUTERS/Carlos Barria)
4. Bernie Sanders, Democrat, senator from Vermont
Sanders had perhaps the best single day of his campaign late last month, romping to landslide victories in Alaska, Hawaii, and Washington. But he got tripped up in New York, with a bigger-than-expected loss to Clinton in her adopted home state.
And he faces challenging odds — he needs to win about 71% of the remaining pledged delegates to overtake Clinton in the pledged-delegate count, according to NBC.
Moving forward, the map doesn't look especially favorable. In a system in which delegates are allocated proportionally, it will be hard for him to make up real ground in large states like California, Pennsylvania, New Jersey, and Maryland.
National polling average among Democratic voters: 46% (2nd) Average in Pennsylvania, Maryland, Connecticut, and Indiana polls: 40% (2nd)
Last month: 4
(Photo by Scott Eisen/Getty Images)
3. Ted Cruz, Republican, senator from Texas
Cruz always had favorable ground in Wisconsin. After that, the map was supposed to get more challenging — and it has.
Late April has overwhelmingly featured Northeast and mid-Atlantic contests more favorable to Trump — including delegate-rich New York, which Trump won with more than 60% of the vote. With more states from the regions on the map to come, Indiana's May 3 primary is looking more and more like a must-win for the Texas senator.
Still, Cruz inspires a flood of enthusiasm among the GOP base, and he may be the best-positioned candidate from within the political sphere to back up the notion that he's not a typical politician, that he is the outsider the base wants despite his day job in Washington.
National polling average among Republican voters: 30.3% (2nd) Average in Pennsylvania, Maryland, Connecticut, and Indiana polls: 24.7% (2nd)
Last month: 3
(Photo via REUTERS/Aaron P. Bernstein)
2. Donald Trump, Republican, businessman
Trump has lit the political world on fire since his entry into the race last summer, and he has showed surprising staying power. We're now on month No. 11 of "The Trump Show."
He has won a majority of contests so far, a feat unthinkable when he entered the race in June. He appears poised to at least enter the convention with the most delegates of any Republican candidate — and after a gigantic win in New York, he seems better positioned to capture the nomination heading into the convention.
There's a clear appetite among Republican primary voters for someone like Trump, who entered the race to controversy surrounding his position on illegal immigration. Business Insider discovered more of that when we followed him on the trail for a week last year.
National polling average among Republican voters: 42.3% (1st) Average in Pennsylvania, Maryland, Connecticut, and Indiana polls: 44.6% (3rd)
Last month: 2
(Photo via REUTERS/Eduardo Munoz)
1. Hillary Clinton, Democrat, former secretary of state
The delegate math is on Clinton's side going forward, especially now that she has avoided her biggest potential pitfall in the New York primary.
The long-presumed Democratic nominee, Clinton has been a shakier-than-expected candidate. But she has a clear look at the nomination, and she would enter the general election with a slight advantage over Trump — or Cruz.
"I'm not going to be responding to him," Clinton said in a recent interview with Business Insider. "I have pretty thick skin. I've been in the arena a long time, and that means that I am not going to get down with him and go insult for insult."
National polling average among Democratic voters: 49.3% (1st) Average in Pennsylvania, Maryland, Connecticut, and Indiana polls: 51.9% (1st)
Last month: 1
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BACK TO SLIDE
Most previous Indiana opinion polls showed a tighter race with Trump leading Cruz by only a few points. A Trump win in the state could be crucial to his chances of securing the nomination but may also offer a gauge of whether he can rally evangelicals.
Cruz, 45, emphasizes his Christian faith on the campaign trail. Focused on Indiana, he says Trump is not an authentic Republican.
A new Cruz advertisement brands Trump and Democratic front-runner Hillary Clinton as two sides of the same coin. "Both support the Obamacare individual mandate. Both support taxpayer funding for Planned Parenthood. Both support letting transgender men go in little girls' bathrooms," it says.
Trump has praised Planned Parenthood family clinics as having helped millions of women even if he opposes funding its abortion component. He backs letting transgender people use the bathroom "they feel is appropriate."
If evangelicals are unenthusiastic, they could sit out the Nov. 8 general election, potentially handing the White House to former U.S. Secretary of State Hillary Clinton, the Democratic front-runner, or her rival, U.S. Senator Bernie Sanders.
"The fear is a lot of them are going to stay home," said Bob Vander Plaats, a leading evangelical activist in Iowa. "You can't win without our base."
Trump rankled some social conservatives by criticizing North Carolina's controversial law that requires transgender people to use the bathroom of the gender of their birth. "The restroom thing is big with a lot of people," said Shan Rutherford, a pastor in Greenwood, Indiana.
Rutherford said he was initially drawn to Trump's message, but was put off by his stance on social issues and his insulting demeanor on the campaign trail. Rutherford, who backs Cruz, said if the White House race came down to Trump versus Clinton, he might not vote.
The Pew Research Center says 45 percent of registered Republican voters nationwide identify themselves as "born-again" or "evangelical." Pew also found that only 44 percent of Republicans view Trump as a religious person. By contrast, 76 percent viewed Cruz that way.
Cruz has assailed Trump for his stance on the North Carolina bathroom law. Trump has said there have been few problems with transgender people using bathrooms. He also has said the question should be left to states to decide.
Cruz charges allowing grown men in bathrooms with little girls is "opening the door for predators."
On Friday, Cruz was endorsed by Indiana Governor Mike Pence, a social conservative who last year signed a bill that critics said could be used by business owners to deny services to same-sex couples.
Trump has said he supports prohibiting companies from firing employees based on sexual orientation, although he has criticized last year's U.S. Supreme Court decision legalizing same-sex marriage.
Cruz has deployed his father, Rafael Cruz, an evangelical minister, as a surrogate in churches in Indiana.
VOTER TURNOUT KEY
A Reuters/Ipsos tracking poll showed 19 percent of respondents would not vote in a White House race between Trump and Clinton. But when filtered to those who attend church nearly every week, the number jumped to 28 percent.
The party cannot afford a low turnout in November. Demographic trends in recent years have favored Democrats, with the pool of Republican voters, overwhelmingly white and older, steadily shrinking.
Eric Houseman, a paramedic from Indianapolis who attended a Cruz rally in Franklin, Indiana, predicted Trump would lose support through such missteps as inaccurately referring in January to a book of the Bible as "Two Corinthians" rather than Second Corinthians or Corinthians II.
"People are beginning to see he's a lot of talk," Houseman said.