Marco Rubio's uneven debate performance just days before the pivotal New Hampshire primary has emboldened a trio of governors seeking to stem his rise in the Republican race for president.
But if Rubio's rivals can slow him in New Hampshire on Tuesday (local time), they are likely to leave the Republican race with a muddled mix of establishment contenders and no clear favourite to challenge outsiders Donald Trump and Ted Cruz.
In the Democratic race, Hillary Clinton found herself trailing Vermont Senator Bernie Sanders by large margins in New Hampshire polls after narrowly winning last Monday's Iowa caucuses.
Clinton aides worry that a big Sanders victory in the state could help him make headway among crucial women and minority voters.
Clinton detoured on Sunday to Flint, Michigan, which continues to deal with the fallout from a lead-contaminated water system, calling on Congress to approve US$200 million in emergency aid to fix the pipes.
Clinton says she is making a "personal commitment" to help Flint, delivering her message at the local Baptist church but also speaking to a more heavily-minority electorate that could help in accumulating enough delegates to secure nomination at the party's national convention.
Sanders, a self-described Democratic socialist, drew another large crowd on Sunday in Portsmouth, where he reprised his indictment of a "rigged economy" and "corrupt campaign finance system."
New Hampshire traditionally hosts the first presidential primary election on the campaign calendar, offering a clue into what Americans want in their next president. The winners will gain momentum heading into the next contests in the more diverse South Carolina and Nevada states.
Among the Republicans, Rubio was downplaying his rough outing in Saturday night's debate, while touting his overall campaign momentum after his third-place finish in Iowa, hoping to use that momentum to boost his chances in Tuesday's contest.
Trump, who finished second to Cruz in Iowa, was pleased with his debate performance and place atop New Hampshire's Republican polls, and again called for the US to reinstitute waterboarding and even harsher treatment for interrogating foreign prisoners.
On NBC's "Meet The Press", Trump said waterboarding, accepted as torture internationally and now forbidden by US law, is "peanuts" compared to what Islamic State group members practice.
Rubio is trying to fend off challenges from New Jersey Governor Chris Christie, Ohio Governor John Kasich and former Florida Governor Jeb Bush.
Cruz, a US Senator from Texas, is not expected to fare as well in New Hampshire as in Iowa, where he drew support from a large bloc of socially conservative evangelicals.
The prospect of Trump or Cruz winning the Republican nomination has set many Republican leaders on edge, and that anxiousness is only likely to increase should New Hampshire voters leave Rubio and the governors clustered together in the primary results, failing to anoint one as their preferred challenger to the front-runners.