The Patriot Act, one of most prominent expansions of federal authority in the past decade, received overwhelming Republican backing in 2006 when it was first up for renewal. This year, even as the surge of the tea party brought to Washington a new legion of lawmakers suspicious of government power, the law again has a good chance of being reauthorized.

The first test comes this month, when three provisions of the act giving the government greater ability to monitor suspected terrorists expire and Congress must decide whether...