ALAN RAPPEPORT

Tension flared in the race for the Democratic presidential nomination Thursday, with each candidate questioning the other’s fitness to lead and Sen. Bernie Sanders pressing his attack that Hillary Clinton is not qualified to be president.

Fresh off resounding victories in Wisconsin by Sen. Ted Cruz and Sen. Bernie Sanders, the presidential campaign shifted Wednesday to New York, where both Donald Trump and Hillary Clinton are trying to seize on their home-state advantage to reverse recent setbacks.

The loss is a significant setback for Marco Rubio, whose fortunes have quickly faded despite his standing as a favorite of the party establishment.

While the Mexican government has said little about Donald Trump’s plan to beef up border security, two of the country’s former presidents have a message for Trump: Mexico won’t pay.

Mainstream Republicans grappled on Sunday with Donald J. Trump’s sweeping victory in South Carolina as if cycling through stages of grief, with some in denial, some outraged and some accepting what now seems increasingly likely.

The backing of Palin, the former Alaska governor and vice presidential nominee, provides Trump with a potential boost less than two weeks before the Iowa caucuses.

Facing new pressure from Sen. Bernie Sanders in Iowa, Hillary Clinton confronted him in direct terms Monday, warning Iowans that, if elected president, he would put their health insurance in the hands of Republican governors and raise taxes on the middle class.

Former Sen. Jim Webb announced Tuesday that he is dropping out of the race for the Democratic presidential nomination after his bid failed to gain traction.

Sen. Bernie Sanders, an independent from Vermont, announced Thursday that he was running for president as a Democrat, injecting a progressive voice into the contest and providing Hillary Rodham Clinton with her first official challenger for the party’s nomination.

Hillary Rodham Clinton on Tuesday defended her exclusive use of a private email address during her time as secretary of state, saying that she did so as a matter of “convenience,” to make life simpler by using one device and one email account.