Senior Republican congressman Steve King has sparked a backlash on social media after tweeting his support for the Dutch anti-Islam politician, Geert Wilders.
"Wilders understands that culture and demographics are our destiny," Mr King wrote on Twitter.
"We can't restore our civilisation with somebody else's babies," he added.
The US Republican representative of Iowa defended his comments on Monday, saying he "meant exactly what he said".
"It's a clear message," he told CNN's New Day programme.
"We need to get our birth rates up or Europe will be entirely transformed within a half century or a little more. And Geert Wilders knows that and that's part of his campaign and part of his agenda."
He would like to see "an America that's just so homogenous that we look a lot the same, from that perspective", he continued.
Mr King is a strong advocate of putting a stop to birthright citizenship.
All children born in the US currently get citizenship under the constitution, including the children of families living in America illegally.
Mr King has pushed for radical reform of the interpretation of the 14th amendment of the US constitution so that it no longer gives the children of undocumented migrants the right to a US passport.
Silence from Republicans - by Anthony Zurcher, BBC News, Washington
It seems Donald Trump is not the only politician who can bring social media to a screeching halt with an inflammatory tweet. Congressman Steve King has a history of walking on the edge of white nationalist rhetoric, and on Sunday afternoon he once again hit the hornet's nest, perhaps in his most direct manner yet.
The outrage from Democratic politicians and commentators across the political spectrum was quick, ferocious and entirely expected. The bluntness of Mr King's message, the talk of "our destiny" and "other people's babies", ensured a vigorous response.
Of greater interest will be how Republican officeholders handle the controversy. So far they have remained silent. That may be increasingly difficult, as this is yet another indication of the growing bonds between the Trump wing of the Republican Party and nationalist movements in Europe.
Breitbart, the conservative media outlet recently headed by White House senior advisor Steve Bannon, often sings the praises of Mr Wilders, as well as France's Marine Le Pen and Frauke Petry, leader of the Alternative for Germany Party.
Mr Bannon has predicted the coming of a new "alt-right" order that will disrupt politics across the West. The question is whether establishment Republicans stay along for the ride.
Mr King's comments in support of Mr Wilders on Sunday led to accusations that he was "openly peddling white nationalism".
His post was retweeted by the former leader of the Ku Klux Klan, David Duke, with the words "sanity reigns supreme".
Mr Duke later tweeted: "God bless Steve King."
But many were quick to denounce Mr King, including US President Bill Clinton's daughter, Chelsea Clinton, who described the Republican's comments as "painful".
The spokeswoman for Republican House Speaker Paul Ryan said Mr Ryan "clearly disagrees and believes America's long history of inclusiveness is one of its great strengths".
Republican and former Florida Governor Jeb Bush also condemned his remarks.
Speaking to the BBC's Adam Smith last year, Mr King said that "millions" of people were expected to enter the US "illegally and unlawfully" in the years to come, with "a birth rate that exceeds that of the American citizen by a factor of two or more".
"That their children would all be citizens would be beyond the pale of the imagination of the people who ratified the 14th amendment," he said.
Mr Wilders, whose populist Freedom Party is expected to do well in Dutch parliamentary elections on Wednesday, has been under 24-hour police protection for more than a decade due to death threats.
He was found guilty of hate speech over his promise to reduce the number of Moroccans in the country last year but no penalty was imposed.
Steve King's controversial remarks
"This whole 'old white people' business does get a little tired, Charlie. I'd ask you to go back through history and figure out where are these contributions that have been made by these other categories of people that you are talking about? Where did any other subgroup of people contribute more to civilisation?" - on MSNBC, July 2016
On undocumented immigrants
"For everyone who's a valedictorian, there's another 100 out there that, they weigh 130lb and they've got calves the size of cantaloupes because they're hauling 75lb of marijuana across the desert." - 2013
On black men
"You know that statistically the greatest danger to a black man in America is another black man." - to Buzzfeed in July 2016
"I had a strong, Christian lawyer tell me yesterday that, under this decision (gay marriage legalisation) that he has read, what it brings about is it only requires one human being in this relationship - that you could marry your lawnmower with this decision. I think he's right." - to Dickinson County News in July 2015