Harkin has not ruled out seeking a sixth term in 2014. And while his vote would likely prevent a primary challenge, it could be tricky for him in a general election.
Republicans - and specifically in the House, where tea party fervor is strong - seem more vulnerable.
While House Republican delegations, such as New York's and Pennsylvania's voted for the bill, they did so likely with impunity because the GOP bases in their states aren't nearly as ideologically conservative as those in other parts of the country.
Wisconsin Rep. Paul Ryan, the 2012 Republican vice presidential nominee, also voted for the measure. It won't likely be an obstacle to his re-election in his swing-voting district, but it could cause him trouble with conservative primary voters, should he run for president in 2016.
Rep. Steve Womack, in just his second term representing heavily conservative northwest Arkansas, could be forced to answer to tea party concerns over his yes vote if he seeks a third term. And he will almost certainly face questions about it should he run for U.S. Senate or governor, the subject of GOP speculation on which Womack has been silent.
Michigan Rep. Fred Upton's backing of the measure might rile up conservatives enough in his right-leaning district in the western part of the state that he could face a challenger. But his stature may be enough to prevent a serious one: he has easily fought off recent primary opponents and, as chairman of the Energy and Commerce commission, would likely have the fundraising edge.
Upton's Michigan colleague, Benishek, also voted for the bill and could have a bigger concern. He eked out re-election to a second term in November, carrying less than 50 percent of the vote in his northern district, and spurning tea party activists there could invite a threat from an opponent.
Among Senate Republicans, Sen. Saxby Chambliss of Georgia backed the measure and may have further agitated conservatives who were already cranky with him over his participation last year in the so-called "Gang of Six," a bipartisan group that discussed fiscal plans including tax increases and changes to entitlement programs.
After the vote, Chambliss pointed quickly to the next phase of the fiscal fight as the place for redemption for what he called a flawed but necessary measure.
Chambliss and others say they will press for tying dramatically lower spending to support for raising the nation's debt limit.
"This is just the first step in a major, major fight," Chambliss' senior adviser Tom Perdue said.
The swift defense from those who backed the increases is a response to GOP primary challenges from conservatives last year that proved costly to Republican members seen as dealmakers. Six-term Indiana Sen. Richard Lugar lost his primary to tea party favorite Richard Mourdock, and House Republicans Jeanne Schmidt of Ohio and John Sullivan of Oklahoma lost in primaries last year, attacked in part for voting to raise the debt ceiling.