Pat Robertson, acknowledging the obvious, endorsed George Bush on Wednesday and said he would drop his own candidacy for the Republican presidential nomination Monday.

President Reagan prepared to make his own formal endorsement of Bush as Democratic front-runner Michael Dukakis celebrated two more big primary victories over Jesse Jackson.Robertson, the former television evangelist, conceded weeks ago that he had no chance to catch Bush. He said Wednesday, after meeting with Bush at the White House, that he wanted to give his supporters a chance to win national convention seats during GOP caucuses this weekend in Arizona and Virginia, then would pull out.

Asked if he would run again in 1992 if Bush loses this fall, Robertson replied, "It's conceivable. It's not a certainty."

Reagan, in a teasing mood, told reporters at the White House - with Bush nearby - "I just have a very brief announcement to make, and that is that I'll have something to say about the campaign tonight." Grinning, he declined to answer questions on the subject.

The president is to announce what everyone already knows at a Republican dinner tonight.

Vice President Bush cruised to barely contested wins in Tuesday's contests in Nebraska and West Virginia. He already has a mathematical lock on the Republican nomination.

But Bush's showing in Nebraska pointed to possible weakness in the fight to come this fall. Sen. Bob Dole of neighboring Kansas, who dropped out of the Republican race almost two months ago, still siphoned off nearly a fourth of the vote.

Even though he has mowed down all his rivals, Bush has made weak showings in some farm states.

With two more wins in his column, Dukakis moved within striking distance of the 2,081 delegates needed to nail the Democratic nomination. According to the latest AP delegate count, Dukakis had 1,581 to Jackson's 942.

In West Virginia, Dukakis captured 254,206 votes, or 79 percent, to Jackson's 45,111 or 14 percent, with 99 percent of the precincts reporting. In Nebraska, Dukakis received 105,542 votes, or 63 percent to Jackson's 43,134 or 26 percent, with 100 percent of the precincts in.

Dukakis won all 37 of the delegates in West Virginia and 18 of the 25 in Nebraska.

Exit polls suggested Dukakis was beginning to make some inroads into Jackson's black support, which has been all but monolithic in contests to date. Only about 4 percent of West Virginia's voters are black, but about a third of them voted for the Massachusetts governor, ABC voter polls on Tuesday indicated.

But Jackson won 24 percent of the white vote in Nebraska, one of his best showings of the primary season among white voters, according to ABC.