The presidential campaign shifted Wednesday to what is now a likely autumn showdown between the big Pennsylvania winners Democrat Michael Dukakis and Republican George Bush.

Dukakis crushed civil rights leader Jesse Jackson by a roughly 2-to-1 ratio in the Keystone State Tuesday, giving the Massachusetts governor his fourth straight major primary victory and making him the runaway favorite to claim the Democratic presidential nomination.Bush, virtually ignored in recent weeks because it was clear he had the Republican nomination sewn up, became a part of the Pennsylvania story by claiming that he believed a landslide win against only token opposition would give him enough national convention delegates to guarantee the GOP nod.

The vice president stopped short Tuesday night of making an unconditional claim to the nomination, planning no formal statement at least until Wednesday.

Dukakis also was cautious, though he emerged from his first head-to-head contest with Jackson bolstered by the unmistakable signal that he is on the road to the nomination. With no major rivals other than Jackson to siphon off votes, the governor's efficient organization appears unstoppable.

"It's a very big boost," he said of Tuesday's victory. Nonetheless, he told a Boston news conference, "We've (still) got to go at it just as strongly and as vigorously as possible."

Former Vice President Walter Mondale, he recalled, "won in New York and Pennsylvania in 1984 and then lost a whole string of primaries. Every one of these is different, and we're going to work hard to win every one of them."

Jackson, already in the next primary battleground of Ohio, refused to concede anything to Dukakis.

"This race is going all of the way until June 7, and our campaign continues to grow," the Chicago preacher declared in Toledo. "We're getting closer and closer every day. We're too close now to turn around."

Despite such rhetoric, the Pennsylvania numbers clearly were bad news for Jackson. With 95 percent of the Democratic ballot counted, Dukakis had 953,539 votes or 67 percent to Jackson's 398,567 votes or 28 percent.

With 94 percent of Republican votes counted, Bush had 644,075 votes or 79 percent. Senate GOP leader Robert Dole of Kansas, who has withdrawn from the race, collected 98,312 votes or 12 percent, and former television evangelist Pat Robertson, who has all but dropped out, had 75,113 votes or 9 percent.