Rep. William V. Chappell Jr., surviving the toughest campaign of his career, won a convincing victory today in a Democratic runoff against wealthy oilman Reid Hughes, who had attacked the seven-term incumbent for being a pro-Reaganomics "Boll Weevil."

State Sen. Van Poole, a 12-year legislator, captured the Republican U.S. Senate nomination to oppose two-term Democratic incumbent Lawton Chiles.

With all the votes counted in the 4th Congressional District, Chappell had 31,226 votes, or 54 percent, to 27,226 votes, or 46 percent, for Hughes.

With 96 percent of the vote counted, Poole had 122,496 votes, or 58 percent, to 88,290 votes, or 42 percent, for three-term Palm Beach County state's attorney David Bludworth in the only statewide race in Florida's runoffs.

While Chappell was backed heavily by conservative business groups and defense-industry officials, Hughes drew support from environmentalists and feminists.

Hughes trailed Chappell by 2 percentage points in the Sept. 7 primary. A liberal political novice, Hughes spent about $250,000 of his own money on commercials that called him the "real Democrat" and attacked Chappell as a traitor to his party who had sold out to President Reagan and to special interests such as defense contractors.

Chappell, 60, said Hughes was trying to "buy" the election with a campaign based on deceit. Chappell lost his Marion County power base when the district was redrawn for this year's reapportionment.

Chappell will face Republican nominee Larry Gaudet, a stockbroker, in the heavily Democratic district in the general election.

Four Republican U.S. House nominations also were at stake.

In the 9th District, one of four created because of population growth, attorney Michael Bilirakis, a political newcomer, upset state House Minority Leader Curtis Kiser. With 84 percent of precincts reporting, Bilirakis had 12,612 votes, or 57 percent, to Kiser's 9,633, or 43 percent. Bilirakis will face state Rep. George Sheldon.

In the 13th District, banker Connie Mack III, grandson of the legendary baseball manager, defeated state Rep. Ted Ewing. With 67 percent of precincts reporting, Mack had 33,043 votes, or 58 percent, to Ewing's 23,856 votes, or 42 percent. Mack will face Democrat Dana Stevens, an economics professor, next month.

In the new 16th District, Port Everglades Commissioner Maurice Berkowitz easily defeated chiropractor Joseph (Doc) Smith. Berkowitz will face state Rep. Larry Smith Nov. 2.

In the 18th District, with 99 percent of the vote counted, Cuban-American Ricardo Nunez defeated Manuel Iglesias in the battle to face 10-term incumbent Claude Pepper, 82. Nunez received 6,725 votes, or 56 percent, to Iglesias' 5,398, or 44 percent.