WASHINGTON (AP) — Senate Republicans united behind Majority Leader Mitch McConnell in insisting that President Barack Obama's successor fill the Supreme Court vacancy after the death of Justice Antonin Scalia. Democrats looking to reclaim the Senate majority immediately accused them of putting politics ahead of their constitutional responsibility.

Vulnerable GOP incumbents in Pennsylvania, Wisconsin, New Hampshire and Ohio — all states that Obama won twice — echoed McConnell's contention that the winner of the presidency in November's election should choose the next jurist. Democrats counter that Obama is president until Jan. 20, 2017, and has every constitutional right to nominate Scalia's replacement.

"I believe the best thing for the country is to trust the American people to weigh in on who should make a lifetime appointment that could reshape the Supreme Court for generations," said GOP Sen. Rob Portman of Ohio.

Said Republican Sen. Pat Toomey of Pennsylvania: "President Obama insists that he will nominate someone for the court. He certainly has the authority to do so. But let's be clear — his nominee will be rejected by the Senate."

Sen. Ted Cruz, the Texan who has practiced before the high court and is seeking the Republican presidential nomination, has made filling Scalia's vacancy the centerpiece of his campaign.

Obama has said he will fulfill his constitutional duty and nominate a replacement in due time. His Democratic allies made it clear that denying the president that right would be an unprecedented step and argued it would enshrine the GOP as "the most nakedly partisan, obstructionist and irresponsible majority in history."

"By ignoring its constitutional mandate, the Senate would sabotage the highest court in the United States and aim a procedural missile at the foundation of our system of checks and balances," Senate Minority Leader Harry Reid, D-Nev., said in an op-ed in Tuesday's Washington Post.

Senate Republicans have the numbers in this consequential "advice and consent" fight.

Republicans outnumber Democrats 11-9 on the Judiciary Committee, which would hold confirmation hearings and vote on whether to send the nominee to the full Senate. The GOP holds the majority, 54-46, and Democrats face an almost insurmountable task in trying to get 14 Republicans to join them in breaking a certain filibuster.

Beyond math is the political calculus. Control of the Senate is at stake this election and Democrats looking to unseat Portman and Toomey — along with New Hampshire's Kelly Ayotte and Wisconsin's Ron Johnson — have seized on their call to wait until next year.

In a fundraising appeal, Ohio Democrat Ted Strickland said Portman "has a clear choice to make: He can look out for his party and D.C. special interests by holding back President Obama's nominee, or he can do his job for the people of Ohio."

In New Hampshire, Democratic Gov. Maggie Hassan criticized Ayotte and argued that Obama's constitutional right to nominate isn't suspended in his last year in office. In Pennsylvania, three Democrats looking to take on Toomey railed against the partisanship over senatorial responsibility.

Democrats are counting on the pressure on Republican senators to force McConnell to allow a nominee to move forward, though the majority leader has shown no signs of relenting since his statement within hours of Scalia's death on Saturday that the vacancy should not be filled until a new president is sworn in.

Various conservative groups which claimed victory when moderate Republican Speaker John Boehner of Ohio stepped down last year, and have questioned McConnell's fealty, made it clear the GOP must stand firm.

"Senator McConnell is right: Under no circumstance should the Republican Senate majority confirm a Supreme Court nominee as Americans are in the midst of picking the next president," said Michael Needham of Heritage Action. "Republican rhetoric condemning President Obama's willful disregard for the rule of law will ring hollow if they do confirm a nominee."

Republicans and Democrats have battled this year over Senate production — or lack of it — as possible action on trade and criminal justice reform look increasingly unlikely. Obama's allies made clear that if a nominee is ignored or delayed, they will accuse the GOP majority of obstruction, and use that argument in their bid to win back the Senate.

"Senate Republicans continue to think that governing is as simple as being against President Obama at every turn," said Sen. Dick Durbin of Illinois, the No. 2 Democrat in the Senate. "It's not, and the American people deserve better leadership than they're getting with this Congress."

One of the most vulnerable Senate Republicans is Durbin's Illinois colleague, Sen. Mark Kirk, who argued that Washington should honor Scalia's life "before the inevitable debate erupts."