State Representative Carl M. Sciortino, one of seven Democrats vying to succeed Edward J. Markey in the US House of Representatives, pressed his opponents in the Fifth Congressional District Monday to oppose the authorization of military action against Syria.

“I am challenging all of my opponents in the Fifth to join me in calling for a no vote on military intervention in Syria,” he said in a morning campaign conference call. “I’m asking all of them to stop waffling.”

He pushed instead for a “political solution through forceful diplomacy.”

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In the hours after the call, three of the other Democratic candidates who, as recently as Friday, had not staked out a firm position on the issue released statements saying they oppose the congressional resolution that would authorize the president to use military force in Syria.

Middlesex Sheriff Peter J. Koutoujian said he does “not support the resolution before Congress at this time.” He cited reservations about the United States’ acting without more international support, as well as the potential for “unforeseen consequences of a strike in that region.”

State Senator Katherine Clark said she does not “think the case has been made that US military action is the best way to move forward, and if the vote were held today, I would vote no.”

State Senator Will Brownsberger said: “I have concluded that, were I already seated in Congress, I would vote against the present resolution authorizing bombing of Syria.”

Sciortino, who has vigorously attempted to position himself to the left of his opponents, was joined on the call by US Representative Alan Grayson, Democrat of Florida, who has endorsed Sciortino and is a vocal opponent of striking Syria.

Sciortino told the Globe last week that if he were in Congress he would vote against authorizing President Obama to use military force in Syria.

Two other Democratic candidates, author Martin Long and Paul John Maisano, who works in the construction industry, have previously said they are opposed to the authorization. Both said Monday that they continue to be in the no column.

State Senator Karen Spilka was the only Democrat in the race who did not take a firm position on the issue, but said in a statement that she is “extremely skeptical of any military action.”

Obama, Secretary of State John F. Kerry, and top congressional leaders have continued to make the case for authorizing a strike after the reported use of chemical weapons by the forces of Syrian leader Bashar Assad.

The president is set to speak to the nation about Syria in a televised address Tuesday.

The Fifth Congressional District, which runs from Winthrop to Woburn to Southborough to Holliston, is heavily Democratic, so the party’s nominee will be the favorite to win.

There are three Republicans vying for the GOP’s nod in the district: Harvard nanophysics researcher Mike Stopa, businessman and attorney Frank J. Addivinola Jr., and actuary Tom Tierney.

Stopa said he would vote in favor of giving Obama authorization to use force while Addivinola would not, according to a spokesman. Tierney did not respond to a voice mail message.

The primary election is Oct. 15, with a Dec. 10 general election. The district became open after Markey won the US Senate seat vacated by Kerry when he became secretary of state.