A letter from Joe Biden to Sir Bill Beaumont, chairman of World Rugby, was part of a finalised World Cup package submitted by USA Rugby in its bid to host the men's event in 2031 and the women two years later.
World Rugby is due to announce the success or not of the US bid on 12 May. Alan Gilpin, chief executive of the governing body, has said World Rugby believes it "can deliver the right outcomes with this hosting plan".
In his letter to Beaumont, Biden wrote: "The United States strongly supports the effort to bring the 2031 Menʼs Rugby World Cup Tournament and the 2033 Womenʼs Rugby World Cup Tournament to our country and looks forward to working with Rugby World Cup Limited to help deliver the most successful Rugby World Cups in history".
The president also pledged "to promote the development of rugby in the United States and worldwide in a sustainable and humanitarian manner, without any discrimination whatsoever, regardless of race, nationality or creed", and says the US government will work to ensure that "any adverse impacts on the environment as a result of the tournaments are minimised".
Biden said governmental guarantees sought by World Rugby would be "executed by officials who have the competence and authority" to do so, or in co-operation with states and private entities. The US will also seek the enactment of any necessary legislation, the letter says.
Biden's Democrats stand to lose control of Congress to Republicans this November. There is however a bipartisan Congressional Rugby Caucus which supports the World Cup bid.
In the formal letter, Biden does not mention his own rugby experience as a player at law school and as a fan, notably of Ireland. The president has often expressed his love for the game and recently hosted a White House visit from the former Ireland and Lions full-back Rob Kearney, a cousin.
After Kearney's visit, when Biden's Democratic predecessor, Barack Obama, made his own White House visit, a signed rugby ball was visible in the Oval Office. In a tweet, Kearney shared a picture of himself giving Biden the ball before "kicking practice in the garden".
In statements accompanying the release of Biden's letter, Jim Brown, the chair of the USA Rugby World Cup bid, said: "We are honoured and humbled that President Biden shares our optimism not only about hosting upcoming Rugby World Cups in the United States, but also about the vast potential the sport has in this country.
"The support of federal, state and local governments is fundamental to the successful planning and execution of a world-class event and this strong endorsement by the president marks a huge step forward in our plans to host incredible Rugby World Cup tournaments in the United States in 2031 and 2033."
Ross Young, the chief executive of USA Rugby, said: "We now optimistically look forward to World Rugbyʼs final decision in less than a month. The potential to grow the sport of rugby in the United States is truly immense, and weʼre all excitedly awaiting next steps should the US be awarded the opportunity to host."
No other host will be announced for the men's event in 2031, should the US not succeed. Australia is set to be named host for 2027. The next men's tournament is in France next year. The US have not yet qualified, needing to beat Chile this summer. The next women's World Cup kicks-off in New Zealand in October.
On Wednesday, USA Rugby also released a list of cities pursuing hosting rights for Rugby World Cup games.
Other cities could be used. The cities listed were: Atlanta, Austin, Baltimore, Birmingham, Boston, Charlotte, Chicago, Dallas, Denver, Houston, Kansas City, Los Angeles, Miami, Minneapolis, Nashville, New York/New Jersey, Orlando, Philadelphia, Phoenix, Pittsburgh, San Diego, San Francisco Bay Area, Seattle and Washington DC.
Washington - or in fact Landover, Maryland, home of the Commanders NFL team - hosted the US Eagles men's team last October. A showpiece game against New Zealand ended in defeat by 104-14 but attracted a crowd of around 40,000.
Bid materials sent to World Rugby alongside the Biden letter, the US bid said, include "a preliminary budget structure, comprehensive data on the candidate host cities and stadiums [and] an initial rugby development and legacy proposal to elevate growth across all levels of the game in the United States".