A Georgia Republican official has said the shooting at a Virginia baseball field will lead to the party's victory in a state special election.
Brad Carver told the Washington Post voters "tired of left-wing extremism" will pick Republican Karen Handel.
Democrat Jon Ossoff and Ms Handel are running for a House seat vacated by US health secretary Tom Price.
Five people were injured last week when a gunman opened fire at a Republican baseball practice.
House Majority Whip Steve Scalise was among those hurt and remains in a critical condition.
The suspect, identified as James T Hodgkinson, allegedly opened fire on Republican lawmakers as they practised for an annual congressional charity baseball game.
It emerged that Mr Hodgkinson, who died of his injuries following a gunfight with police, was a volunteer for Senator Bernie Sanders' presidential campaign and promoted anti-Republican rhetoric on social media.
Mr Carver, the Republican Party chairman in Georgia's 11th Congressional District, told the Post "the shooting is going to win this election for us".
He went on: "Moderates and independents in this district are tired of left-wing extremism. I get that there's extremists on both sides, but we are not seeing them."
Stakes are high - Anthony Zurcher, BBC News, Washington
If November was the height of political winter for Democrats, the party faithful have been desperately searching ever since for the first signs of spring. In special congressional elections in Kansas and Montana they've strained to see green shoots. Narrow defeats there, with talk of moral victories, have only gone so far.
In Georgia on Tuesday, if Democrats hope for a thaw, they need to post a win.
The Atlanta-area race is the kind of contest Democrats need if they want to take back control of the House or Representatives next year. It's a historically Republican seat that Donald Trump barely won last November, full of the educated suburban voters who populate key swing districts in Florida, California and across the South.
Mr Trump has celebrated his party's previous special election victories, and on Monday he tweeted out support for the Republican candidate in Georgia. Democrats would love to give him a taste of electoral defeat.
If they come up short, frustration - tinged with hopelessness - will follow. If they win, Republicans will look to next year's balloting with foreboding bordering on panic.
It's one election; a single seat out of 435. The stakes, however, are just that high.
The much-anticipated election in the Atlanta suburban district between Ms Handel and Mr Ossoff is seen as a test of President Donald Trump popularity.
The showdown is also considered the most expensive US House of Representatives race in the nation's history.
Mr Carver praised Mr Ossoff for running a "brilliant campaign", but contended the baseball shooting would tip the outcome in Republican favour.
"We're seeing absolute resistance to everything this president does," he added.
"Moderates and independents out there want to give him a chance. Democrats have never given this president a chance."
His comments came as a Republican group released an advertisement attempting to link the congressional shooting to Mr Ossoff's Democratic campaign.
The television spot, produced by the Principled PAC, showed an image of Mr Scalise on a stretcher after the attack.
"The unhinged left is endorsing and applauding shooting Republicans," a narrator is heard saying in the ad.
"When will it stop? It won't if Jon Ossoff wins on Tuesday."
The ad, which includes an image of US comedian Kathy Griffin holding a fake severed head that looks like Mr Trump, claims "these same leftists are all for Ossoff, and he wins, they win".
Both Mr Ossoff and a spokeswoman for Ms Handel condemned the spot, the Atlanta Journal-Constitution reported.