GRAND JUNCTION, Colo. (AP) _ Rep. Tim Wirth, who has Gary Hart's endorsement for the Senate seat Hart is vacating, has begun a statewide fund-raising tour but has not announced formally that he will run.

Wirth, D-Colo., kicked off the weeklong tour Sunday in Grand Junction, sauing he will schedule a formal announcement in about two weeks about whether he will seek Hart's seat.

''This trip gives me the chance to think of the Senate race much more seriously,'' Wirth said.

Hart announced Saturday in Evergreen that he will not seek a third Senate term, hinting strongly that he will pursue his party's presidential nomination in 1988.

''I won't be coy about my plans,'' said Hart. ''Does that mean I'm making some announcement about 1988? Nope. Does it mean I still have an interest in being president? Yep.''

Hart, whose unsuccessful 1984 presidential bid left him with more than $3 million in still-unpaid campaign debts, has maintained he would not commit to a second presidential race until sometime in 1987.

Hart became the front-runner for the 1988 nomination when Sen. Edward Kennedy, D-Mass., declared two weeks ago he would not seek the presidency.

The role is an enviable one, but one not without its pitfalls.

''I think it is the most hazardous position a political figure can be in,'' said George McGovern, who ran for president in 1972 with Hart as his campaign manager.

''I had no problem in 1972 with the other Democrats until I became the front-runner, and then all of a sudden I got brickbats in every direction - from the left, from the right, from the middle, across the political spectrum. You are so much more the target of criticism and your mistakes are so much more magnified than if you are down in the pack.''

In a discussion on CBS-TV's ''Face the Nation'' on Sunday, Gov. Charles Robb, D-Va., and Gov. John Sununu, R-N.H., agreed that Hart and Vice President George Bush were their parties' leaders in the 1988 race.

Robb agreed that ''that is a difficult position to maintain. We have a whole stable of emerging leaders within the Democratic party that can and want to challenge for that position.''

Other Democrats whose name arise in discussions of the next presidential campaign include Gov. Mario Cuomo of New York, Sen. Joseph Biden of Delaware and Rep. Richard Gephardt of Missouri.

In 1984, Hart began his campaign for the Democratic presidential nomination as a dark horse - stressing his ''new ideas'' platform - but gave Walter Mondale a strong challenge before the latter won the nomination.

Some Hart supporters had urged him to seek re-election to the Senate, believing a strong victory could only enhance his presidential prospects. But others said a Senate race and a subsequent presidential bid would have made him susceptible to charges from Republicans that he had abandoned his commitment to Colorado. Hart had a poor Senate attendance record - 34 percent - in 1984.

Hart said he would support Wirth for the Senate. Wirth, considered the leading Democratic contender for the seat, is known nationally for his work as chairman of the House panel which oversaw the breakup of AT&T.

Three Republicans already have begun campaigning for Hart's seat: Rep. Ken Kramer, state Sen. Martha Ezzard, and Terry Considine, son-in-law of state GOP Chairman Howard ''Bo'' Callaway.

In making his announcement Saturday, Hart said the decision was not an easy one.

''It would be tempting to look forward to defeating the far-right wing again and telling them to pack up their pathetic political baggage and get out of town,'' he said.