The centre-left lawmaker said however that the observers are absent from some states, for example Texas and Arizona.
The chief coordinator of the OSCE's beefed-up election observer mission in the United States denounced on Tuesday as "wild speculation" Republican presidential candidate Donald Trump's claims that the vote is rigged.
"Such wild speculation is very bad for politics," Christine Muttonen of the Organization for Security and Co-operation in Europe told Austrian public radio station Oe1, saying it leads to people "turning their backs" on the political process.
"Democracy is always damaged when there are unfair, bad-tempered (and) very polarising comments and speculation," the Austrian said in an interview from Washington broadcast ahead of the opening of polling stations later Tuesday.
The Vienna-based OSCE, of which the United States is a member and which monitors elections worldwide, is sending 300 observers from 43 countries, up from just 57 in 2012. Muttonen said the increase was due to the "very large interest".
The centre-left lawmaker said however that the observers are absent from some states, for example Texas and Arizona. Some states have legislation barring international observers, she said, and "prefer to do it themselves".
"This contradicts of course this commitment signed by all OSCE members that international election observers are welcome," Muttonen said.