Washington made a formal request in January for permission to place a radar base in the Czech Republic and 10 interceptor missiles in Poland as part of a European defense shield U.S. officials say is needed to protect against a threat from Iran.
The center-right government of Prime Minister Mirek Topolanek supports the plan and wants Parliament to vote on the request.
Paroubek's Social Democrats oppose the plan and have demanded a referendum on the issue.
Paroubek said the Iranian threat is a "virtual reality" now and it would take seven to 10 years for Iran to develop such weapons.
But "if in the future the (Iranian) ballistic missiles are there, we are ready to modify our position," he said.
Paroubek, the former prime minister who is chairman of the Social Democratic Party, is in Washington for talks with U.S. defense officials and Democratic lawmakers.