WASHINGTON — Senate Republicans threatened today to block virtually all legislation until expiring tax cuts are extended and a bill is passed to fund the federal government, vastly complicating Democratic attempts to leave their own stamp on the final days of the post-election Congress.

“While there are other items that might ultimately be worthy of the Senate’s attention, we cannot agree to prioritize any matters above the critical issues of funding the government and preventing a job-killing tax hike,” all 42 GOP senators wrote in a letter to Majority Leader Harry Reid, D-Nev. The 42 signatures are more than enough to block action on almost any item he wishes to advance.

The threat does not apply to a new arms control treaty with Russia that is pending, since it would be debated under rules that differ from those that apply to routine legislation. President Barack Obama has made ratification of the pact a top priority.

But it does threaten Democratic attempts to lift the Pentagon’s ban on openly gay members of the military, and a separate item to give legal status to young illegal immigrants who attend college or serve in the military. The tax and spending bills are likely to be the last to pass before Congress adjourns for the year.

“Republicans have pleaded with Democrats to put aside their wish-list to focus on the things Americans want us to focus on. They’ve ignored us. The voters repudiated their agenda at the polls. They’ve ignored them. Time is running out. They’re ignoring that,” Senate Republican Leader Mitch McConnell of Kentucky said in remarks on the Senate floor. “The election was a month ago. It’s time to get serious. It’s time to focus on priorities.”

McConnell and Reid were expected to discuss the legislative agenda later in the morning in a meeting that was previously scheduled.

The Democratic to-do list also includes extending the expiring tax cuts – although they and Republicans differ on particulars, as well as a measure to keep the government in operation. But the rest of their agenda marks an attempt to court voters Democrats need in 2012 to recapture the majority.

Call it lame-duck politics.

Take the so-called Dream Act, a measure to give young people whose parents brought them into the United States illegally before they were 16 a path to legal status by going to college or joining the armed forces.

The measure has enjoyed some degree of bipartisan support in the past, and Reid, the majority leader, vowed last month – in the thick of his tough re-election fight in heavily Hispanic Nevada — to hold a vote on it when Congress returned to finish its end-of-the-year business. He said Tuesday he’d move to overcome GOP objections and force a test vote, although it’s unclear when one will occur.

Hispanic voters also played a major role in sparing other Democrats – including Sens. Michael Bennet of Colorado and Barbara Boxer of California – from being toppled by a GOP wave.

“There was a firewall in the West where Latino voters turned out in big numbers to reward people who championed them,” said Frank Sharry of America’s Voice, an immigrant advocacy group. “We’re going to try to make it painful” for those who oppose efforts to give illegal immigrants a path to legal status, he added.

Most Republicans vehemently oppose the Dream Act, saying it amounts to amnesty.

Hispanic groups have pushed hard for the legislation, which targets the most sympathetic of the millions of undocumented people – those brought to the United States as children, who in many cases consider themselves American, speak English and have no ties to or family living in their native countries.

Miffed that Obama has not done more to press for a broad immigration overhaul to give undocumented people a path to legal status, the groups say the least Democrats can do is push through the more limited bill for young people.

Republicans decry the strategy of acting on such issues during the lame-duck session, accusing Democrats of playing politics and ignoring the message voters sent Nov. 2.

But Democrats also face pressure from their left flank.

Gay-rights groups have criticized Reid for not pushing hard enough to repeal the “don’t ask, don’t tell” policy against openly gay soldiers, as the House has already voted to do.

Reid has promised to hold a Senate vote on the matter before year’s end, after hearings can be held later this week on a Pentagon report on the impact that openly serving gays would have on the military.

Republicans say they need to examine the report, which was issued Tuesday, before acting. It concluded that getting rid of the policy might cause some disruption at first but wouldn’t create widespread or long-lasting problems.

Obama seized on the conclusion to call on the Senate to act “as soon as possible” to repeal the ban, “so I can sign this repeal into law this year and ensure that Americans who are willing to risk their lives for their country are treated fairly and equally.”