Welcome to The Tip Sheet, a daily political analysis of the 2018 elections, based on interviews with Republican and Democratic officials, pollsters, strategists and voters.

Where things stand

• For a sign of why Republicans are starting to think they might — might — have an outside chance of keeping control of the House, look at our new poll of an Illinois congressional race that has been considered a tossup. The G.O.P. incumbent, Mike Bost, has a nine-point lead over the Democrat, Brendan Kelly, but in early September the two men were running about even, according to New York Times/Siena College polls.

What changed? President Trump’s approval ratings. Voters in that district, Illinois’s 12th, were roughly split on him in September; in the new poll that wrapped up Monday night, 50 percent approved of Mr. Trump’s performance and 43 percent disapproved.

• Republicans generally know which districts they need to defend to keep power. But strategists for the party, which can afford to lose a net total of 22 seats, are growing concerned about two open seats that were not expected to become so competitive — and outside conservative groups are considering pouring advertising dollars into both this week.