Toward the end of their first debate Monday night, Sen. Lowell P. Weicker Jr. (R-Conn.) glanced to his left and then to his right and observed that one challenger seemed to be saying, "I'm sabotaging Ronald Reagan," and the other that, "I'm a closet Reaganite."

The truth, he said with a smile, is that, "I'm a senator who thinks for himself, and I'm not going to change. I'm going to keep telling it like it is."

Weicker is starting his fight for a third term just where he likes it best and seems to be most often -- in the middle of a cross fire of controversy.

With Rep. Toby Moffett, the aggressive Democratic challenger linking him to administration economic policies that have "crushed the dreams" of thousands of jobless workers, and Conservative Party hopeful Lucien P. DiFazio Jr. calling him a "chameleon... who has put the knife in the back of your party," Weicker would seem to have precious little safe ground.

One reason that he is rated one of the shakiest Republican incumbents in the country is that a series of polls over the past year have consistently showed him with less than 50 percent support.

The most recent poll, published Sunday in the Hartford Courant, showed that with DiFazio's late entrance into the race, it was Weicker, 31 percent; Moffett, 30 percent, and DiFazio, 5 percent, with the rest undecided. A fourth candidate, Libertarian James A. Lewis, made little impact on the television debate and did not register in the poll.

Republicans claim to have private polls showing a slightly wider margin for Weicker, but not enough to take him off their worry-list.

But as the fight begins in earnest, there is a growing private belief in both parties that Weicker may hold the winning cards.

Some of this is due to the money advantage that the incumbent, an heir to the Squibb pharmaceutical fortune who advanced his own campaign almost $200,000 in start-up funds, may command down the stretch. Part is due to the organizational uncertainties that caused Moffett, a four-term House Democrat best known for his championship of consumer causes, to change managers and media advisers in late summer.

And part of it is the questionable staying power of DiFazio, a handsome, articulate lawyer who has run unsuccessfully six times for lesser offices, as a Democrat and Republican.

But mostly, it is the feeling among both critics and supporters that Weicker is a man who fights best with his back to the wall.

Three months ago, there was doubt whether he would even make it to the general election. The maverick Republican, who had been uninhibited in his criticism of the last three GOP presidents and, often as not, had shunned other Republican office-seekers in Connecticut, was trailing Prescott Bush Jr., brother of Vice President Bush, in a prospective Republican primary. But Weicker trounced Bush so badly in the July Republican endorsing convention that Bush did not even carry his challenge to the primary.

Now, Weicker looks to be on a roll. He has just come off successful Senate floor fights in which he played a key role in blocking school prayer and antiabortion amendments, winning praise for "a fabulous job" from Mr. Conservative himself, Sen. Barry Goldwater (R-Ariz.).

As if that were not praise enough, today's Hartford Courant also carried a story that the Appropriations defense subcommittee on which Weicker serves had given Pratt & Whitney, a major local contractor, $54 million for advance work on aircraft engine design.

Such developments blunt the thrust of Moffett's two main lines of attack. In Monday's debate, the Democrat said that for all his vaunted independence, Weicker as a Republican keeps such conservatives as Jesse Helms, Orrin G. Hatch and Strom Thurmond in their committee chairmanships and "cuts his deals" with them.

"That's funny," Weicker said, "'cause every time I look up on the Senate floor, I'm fighting Helms, Hatch and Thurmond. I'm the skunk at their garden party."

And when Moffett charged that Weicker is an ineffective loner, a man who does not even attend the delegation breakfasts to work on state problems, Weicker said, "I'm not going to get contracts out of delegation meetings. I get them on my defense Appropriations subcommittee."

A fellow-Democratic officeholder commented after the debate, "Moffett's campaign is unfocused. It's too easy for Weicker to dispose of his charges."

Because of Weicker's frequent support for union positions and his differences with the American Federation of State, County and Municipal Employees union and its late president, Jerry Wurf, Moffett is unlikely to get the two-thirds vote needed for endorsement by the state AFL-CIO later this week.

But the young congressman has the backing of the United Auto Workers and the Amchinists. A onetime Nader's raider, Moffett also is beginning to build a statewide machine of youthful organizers similar to the one that made him unbeatable in his normally marginal House district.

Ironically, he may also be helped by the strong lead in the polls for incumbent Gov. William A. O'Neill (D). Moffett gave serious thought to challenging O'Neill, but lately has been attempting to cozy up to the Democratic "regulars" represented by the governor.

But he is also hoping for an assist from DiFazio, who petitioned his way onto the ballot just 10 days ago and says he hopes for $300,000 from Connecticut conservatives, the National Conservative Political Action Committee's Terry Dolan and New Right direct-mail specialist Richard Viguerie.

DiFazio describes himself as the only Reagan supporter in the race, and Moffett strategists say that if he can take 10 percent or more of the vote, the middle ground Weicker is trying to occupy will get too narrow to elect him.