The narrow victory by Ron Wyden, a liberal congressman, came in the first US election conducted entirely by post. It was held to find a successor to Bob Packwood, who resigned in disgrace from the Senate last summer after charges of sexual harassment. Mr Wyden defeated his Republican opponent, the conservative businessman Gordon Smith, by a margin of 17,000 votes, or 1 per cent of the total ballot.
Despite the closeness of the outcome, the White House and Democratic strategists were exultant yesterday, hailing it as further proof of the unpopularity of the Republican-controlled Congress, and another sign that President Clinton will win a second term. The result gives the Democrats 47 senators to the Republicans' 53.
More heartening still for Democrats, Mr Wyden pitched his campaign on the very issues Mr Clinton will highlight this autumn - the protection of education, the environment and free medical services for the poor and elderly, and a "safe, legal and rare" approach to abortion.
Oregon, moreover, continues the trend visible at the off-year elections of November 1995, when the heavily favoured Republicans failed to seize control of state legislatures in Virginia and Maine and the governorship of Kentucky.
The Wyden win is another pointer that the American public feels that the congressional conservatives led by Newt Gingrich, the House Speaker, have gone too far, especially in their intransigence in the budget dispute, which caused two government shutdowns.
The Oregon poll also seems to vindicate postal voting, permitted in the three-week period before election day. Those who preferred to wait until Tuesday itself could leave their ballots in 160 "drop-boxes" across the state which were sealed at the 8pm deadline.
Despite predictions that public interest would plummet, turn-out topped 60 per cent, compared with 41 per cent in the 1994 congressional elections, and exceptionally high for a by-election. Oregon saved $1m (pounds 660,000) on the exercise.
The ranks of congressmen not seeking re-election swelled further yesterday, as Charlie Rose, the influential North Carolina Democrat and a champion of the beleaguered tobacco industry, announced his retirement after 24 years. Life in the minority was "not as much fun", he said. He is the 38th House member and 25th Democrat to step down. A record 13 senators have announced they will not seek re-election, eight of them Democrats and five Republicans.