Warning that a second-term Jimmy Carter would turn more conservative, Rep. Morris Udall (D-Ariz.) yesterday endorsed the presidential candidacy of his old friend and fellow liberal, Sen. Edward M. Kennedy (D-Mass.).

Udall, Carter's most dogged opponent for the 1976 Democratic nomination, expressed surprisingly little quarrel with the president. He praised Carter's foreign policy and his handling of the Iranian crisis. "The president has been fair to me. I have no complaints about him on a personal basis," Udall said.

But he told a breakfast meeting of reporters, that "on a range of issues, Sen. Kennedy's positions are much closer to mine" than are Carter's and Kennedy would be a more effective national leader. He warned, that "the more conservative Jimmy Carter would come out," if the president won a second term.

He also argued that Kennedy's nomination would boost the chances of Deomcratic candidates in Arizona and around the country.

The endorsement comes at a time when Carter has drawn ahead of Kennedy, whom he trailed by a 2-to-1 ratio a few months ago, in public opinion polls. It was seen as an effort to shore up Kennedy's sagging support among liberals.

Udall downplayed the importance of Carter's sudden surge in the polls. "A president who can go up 20 points in two weeks can go down 20 points in two weeks," he said.

He also defended Kennedy's spotty performance on the campaign trail. "This is Sen. Kennedy's first national campaign. It's trial by fire," he said, adding later, "He's really never had a tough fight in Massachusetts. They've elected him by acclamation there every six years."

Iowans apparently are not as bothered by Ronald Reagan's age as they used to be. A new poll by the Des Moines Register found that 27 percent of the people in the state, site of the first formal matchup in the 1980 presidential race, now think the former movie actor is too old to be elected president. In August, 38 percent thought he was too old. Reagon is 68.

The same poll found that Iowans are not satisfied with Kennedy's explanation of the Chappaquiddick incident. More than half (52 percent) of those polled said they do not believe Kennedy. Only 23 percent of the people believe him.

A fund-raising letter authorized by Sen. Herman Talmadge's campaign committee warns recipients about two liberal senators who would succeed Talmadge in key committee posts if he loses in 1980.

The campaign solicitation letter tells recipients that the defeat of Talmadge, a 23-year Senate veterans, would make Sen. Gaylord Nelson of Wisconsin chairman of the subcommittee on health and Sen. George McGovern of South Dakota chairman of the Agriculture Committee. Both are now chaired by Talmadge.

Talmadge faces a tough reelection campaign after being denounced by the Senate last fall for "reprehensible" handling of government funds. The action followed 16 months of investigation by the Senate Ethics Committee, which found that Talmadge's office made $43,435 in false expense claims, filed false campaign financing reports and failed to report more than $10,000 in campaign contributions.

Vice President Mondale called GOP presidential hopeful Sen. Howard H. Baker aside the other day to talk about Kennedy's floundering presidential campagin. Mondale asked Baker if he had seen Kennedy lately.

"I just passed him in the hall," Baker replied.

"Well, if you see him again, tell him to keep doing whatever he's doing, Mondale said.