After 10 weeks of bashing Walter F. Mondale from coast to coast as a big-spending "doomsayer," Vice President Bush switched targets today and took out after House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

In the wealthy suburbs of this Democratic city, Bush kicked off an effort to increase the GOP's strength in the House on the coattails of an anticipated landslide by President Reagan.

Bush plans to take the coattails crusade this week to 15 congressional districts in Alabama, Mississippi, Illinois, Ohio, New York, Connecticut, Delaware and New Jersey, stumping for Republicans who are believed by national strategists to be within striking distance of victory if given a boost from the national ticket.

"We need Jabo Waggoner in the House. We need him bad!" Bush declared to several hundred screaming teen-agers and a handful of adults in the sweltering gymnasium of Vestavia Hills High School.

"It's a big job, but we have to lift up Tip O'Neill and move him over with those liberal Democrats and send men like Jabo Waggoner" to replace them, Bush said.

In Waggoner's case, the coattails strategy does not look promising. A Democratic state legislator who switched parties for this election, Waggoner is trailing Rep. Ben Erdreich (D-Ala.) in most polls by more than 20 points -- about the same amount that Reagan is leading Mondale in Alabama.

Erdreich, who upset an incumbent Republican in 1982 with a coalition of black and blue-collar supporters, has voted against the Democratic leadership on key issues, such as the MX missile, and is campaigning on his independence.

Seeking to capitalize on Waggoner's recent GOP conversion, Bush told a local television station: "I think it's happening to many people in Alabama, just like many people in my state of Texas who have been Democrats all their lives, but say I just can't identify with the party of Mondale-Ferraro, with the party of Tip O'Neill." Bush also campaigned today for Sonny Callahan, another former Democrat seeking the open seat of retiring Rep. Jack Edwards (R-Ala.) in Mobile. Other challengers scheduled to receive a boost from Bush this week include:

David Armstrong against freshman Rep. Wayne Dowdy (D-Miss.); Ken McMillan against freshman Rep. Lane A. Evans (D-Ill.), Paul Aniboli against Rep. Thomas J. Downey (D-N.Y.); Robert Quinn against freshman Rep. Robert J. Mrazek (D-N.Y.); Jill Emory against Rep. Stan Lundine (D-N.Y.); Frank Venner against freshman Rep. Marcy Kaptur (D-Ohio); Larry Denardis against freshman Rep. Bruce A. Morrison (D-Conn.); Elise Dupont against Rep. Thomas R. Carper (D-Del.), and Dean Gallo against Rep. Joseph G. Minish (D-N.J.).

Bush is also scheduled to campaign for GOP incumbents Rep. Bill Green (R-N.Y.); Rep. Dan Crane (R-Ill.); and Rep. William Carney (R-N.Y.), and for Joe DioGardi, who is in a close race for the open seat of retiring Rep. Richard L. Ottinger (D-N.Y.).