President Reagan, buffeted by the Iran-contra affair and Democratic opposition in Congress, vowed yesterday to carry his "crusade" into the 1988 campaigns and said his presidency has not run out of steam.

"All those who talk about lame ducks and the post-Reagan era are dead wrong," the president said in a speech to a group of Republican loyalists. "I can't help but noticing that they're the same gurus whose mantra for the last six years has been, 'This too shall pass.'

"There will be no post-Reagan era," he said, "because there's been no Reagan era in the way those people mean it. This has been the era of the American people. Leaders may come and go. When it comes to the American people, their truth keeps marching on."

Answering criticism that his administration lacks a fresh agenda, Reagan said, "The challenge for us today is to use the next year and a half to secure the progress that we've made so far . . . .

" . . . Some in Congress want to cut defense spending, raise taxes and go back to the old days of big government. I won't let them," he said to applause from the group, which included 1980 and 1984 campaign veterans.

Reagan again criticized the budget resolution approved by the Democratic-controlled Congress and said spending bills that come to the White House and reflect priorities in the resolution will be "dead on arrival."

While critics say Iran-contra disclosures have damaged Reagan's case for new funding for the Nicaraguan rebels, the president said, "Nothing has been said in the last six months that changes one basic truth. For our nation, and for the cause of freedom and democracy around the world, America must stand by the freedom fighters in Nicaragua."