One of the first acts of the next Congress should be to outlaw lame duck sessions. The voters just fired the Democratic majority in the Senate, after all, and replaced them with a Republican majority that campaigned against Obamacare and big spending. But the GOP majority doesn’t take office until January. Meantime the Democrats who lost are still running the show.

These lame ducks lack the moral authority to govern. They shouldn’t be allowed to do any more than the bare minimum to keep government operating until January, when the new Congress meets. Allowing the majority leader, Harry Reid, to ram through an omnibus spending bill for the coming year, or make other key decisions, would be like letting your ex-spouse keep using your checkbook.

Lame duck sessions were unavoidable before jet planes. The framers of the Constitution provided 17 weeks for newly elected lawmakers to travel and take their seats on March 3. That was the 18th century. They didn’t even have the Internet. In 1933, Americans ratified the 20th amendment to eliminate lame duck sessions. Except in times of war, Congress didn’t meet after the November elections. But for the last two decades, lawmakers have weaseled around the amendment’s intent, returning after election day to deal with issues they avoided before election day.

That’s our situation now. The lame ducks are in charge, poised to act without fear of voter reprisals. That means poised to spend, spend, spend. The federal government will run out of money on December 11, unless Congress approves new funding to avoid a government shut down. Mr. Reid will ask for an omnibus spending bill good until September, 2015 , gilded with a whopping $6.2 billion for Ebolacare, $5.6 billion for expanded warfare against ISIS, $3 billion for an international climate change fund, and other gargantuan requests.

Republicans should say no, and offer instead to fund government at current levels for another two or three months. No longer. Allowing spendaholic Democrats to stay in charge of the nation’s cash register until next September would betray the very voters who just ousted them from power.

Giving the lame duck Mr. Reid his omnibus bill would also diminish the new GOP Senate majority ‘s bargaining power to start dismantling Obamacare. Total repeal may not be in the cards as long as President Obama is in office, but GOP lawmakers could refuse to appropriate money to write more Obamacare regulations and might press Mr. Obama to go along with repealing the job-killing employer mandate in return for spending concessions.

Most important, saying no to the lame duck omnibus bill puts the next Congress in a better position to resist Mr. Obama’s attempts to govern with his pen and phone, evading Congress.. The president says that in the coming days, he will announce bold changes to the nation’s immigration laws. Speaker Boehner asked Mr. Obama to wait for Congress to act. Mr. Obama says Congress waited too long. The president’s pretensions to one-man government alarm even those who favor his views on immigration.

Article 1 says all legislative powers are vested in Congress. It doesn’t say “unless Congress waits too long.” If Obama grabs more power than the Constitution allows, the new Congress’s best bargaining chip will be control over spending. James Madison, chief architect of the Constitution explained in Federalist No. 58 that withholding funds — not impeachment or lawsuits — would be the “most complete and effectual weapon” to stop a presidential power grab.

That’s why the GOP must not acquiesce to Mr. Reid’s parting request to fund the government until next September. His lame duck majority should do be the last in history. When Senator McConnell, the presumptive Majority Leader, takes charge in January, he should introduce a bill to bar Congress from meeting between election day in November and the swearing in of newly elected members in January. That simple change will put an end to lame duck sessions and put voters where they belong: in charge from election day forward.