Why Sen. Cranston is dubbed `most vulnerable Democrat'

It's a four-hour, 2,000-mile flight from Washington, D.C., to California, but almost every weekend United States Sen. Alan Cranston makes the grueling round trip. There is good reason for Senator Cranston's attentiveness to his home state. The senator, who once set a world's record (12.6 seconds) in the 100-yard dash for people over 55, today finds himself in an even more challenging race.

Republicans have dubbed Mr. Cranston, who is seeking his fourth term, ``the most vulnerable Democrat in the Senate.'' Polls show his public support has waned.

That's wonderful news for the GOP, trying to cling to a slim 53-47 margin in the Senate. A Democratic loss in California could ensure GOP control of the Senate for the rest of President Reagan's term.

Ironically, many of Cranston's problems in the California election this year have little to do with himself. One of the most worrisome elements for the senator is the ``Rose Bird factor.''

Chief Justice Bird, who heads the California Supreme Court, must go before the voters in November to be reconfirmed. She has become a controversial figure because of her opposition to the death penalty.

She could draw tens of thousands of anti-gun-control, pro-death-penalty conservatives to the polls, and most of them would probably vote against Cranston.

The senator also has created some of his own difficulties.

Pollster Mervin Field says Cranston's troubles began when he made his abortive 1984 bid for the Democratic presidential nomination.

Ordinarily, voters are pleased to see one of their local heroes run for higher office. But in Cranston's case, it backfired. The senator, trapped in a crowded field of middle-of-the-road candidates in 1984, moved to the left to find some running room and based his campaign on the peace movement.

Cranston had always drawn heavy support from California moderates. But his nuclear-freeze crusade made these voters see him with new eyes. His standing at home plummeted, Mr. Field notes.

Since then, there have been other troubling developments.

In past elections, Republicans have always inadvertently cooperated with Cranston by nominating right-wing candidates. That left the political center open to the senator. This time, the GOP picked Ed Zschau, a moderate-to-liberal congressman who draws his greatest strength from the center.

Furthermore, Mr. Zschau has won the backing of California big business, which in the past has lined up with Cranston. As a result, Zschau will have a rich political war chest that could reach $12 million, enabling Republicans to flood the TV airwaves with pro-Zschau, anti-Cranston commercials.

An additional element working against Cranston might be called the ``Pete Wilson factor.'' Political insiders say that Republican Senator Wilson, the junior member of the California senatorial duo, has been extremely effective in getting things done in Washington.

In the past, Californians who wanted action on a problem had to turn to Cranston. But now, with Mr. Wilson's party in control of the Senate, they have found an alternative with the junior member.

Dislodging an incumbent like Cranston remains an uphill task. Polls have shown Cranston consistently ahead of every Republican seeking the office, including the primary winner, Zschau. Yet Republicans still find hope in the polls. The Field poll, for example, shows Cranston with an ``excellent or good'' approval rating among only 37 percent of California voters -- unusually low for an incumbent. By contrast, California Gov. George Deukmejian, a Republican, rates 57 percent. President Reagan scores 60 percent.

Faced with all this, Cranston plans an expensive counterattack. Only hours after Republicans picked their candidate, Cranston was on TV with anti-Zschau commercials.

His sharpest charge so far: that Zschau is a ``flip-flopper'' on such crucial issues as the MX missile and aid to the ``contras'' in Nicaragua.