No, \\
in normal LR mode will not switch to vertical mode, nor does \newline
.
In normal LR mode, the definition of \newline
is \@normalcr\relax
; the \relax
is meant to avoid a subsequent *
to be interpreted as a *-variant.
The definition of \\
is essentially \@normalcr
, which however looks for a following *
and an optional argument. If the optional argument is present, it should be a length and LaTeX inserts \vadjust{\vskip<length>}
, which becomes \vadjust{\nobreak\vskip<length>}
if a *
followed the \\
, then inserts \hfil\break
to trigger a line break. The effect is to add a vertical space between the broken line and the following one; no page break can occur in case \\*
or \\*[<length>]
is used. In the former case, only \vadjust{\nobreak}
is added, of course.
The \vadjust
item adds vertical material which will be considered during page setting, after the paragraph has been broken into lines and the boxes so obtained are contributed to the vertical list under construction.
The possible added glue will be inserted between two lines in addition to the normal interline glue.
In general, the interline glue between two lines of a paragraph is a feasible page break point, unless the interline glue is preceded by a penalty, that may be inserted automatically: \clubpenalty
between the first two lines, \widowpenalty
between the last two lines and \interlinepenalty
between any other pair (if the parameter is not set to 0).
What you want can be easily obtained by
\documentclass{article}
\usepackage{booktabs}
\usepackage{lipsum}
\begin{document}
\lipsum[2]
{\nopagebreak\tiny\noindent\rule{\textwidth}{\heavyrulewidth}\\*
\tiny Here is some text.\par}
\lipsum[3]
\end{document}

However, a simpler method is
\documentclass{article}
\usepackage{booktabs}
\usepackage{lipsum}
\begin{document}
\lipsum[2]
\par\kern3pt
\hrule height \heavyrulewidth
\kern1pt
{\tiny\noindent Here is some text.\par\nointerlineskip}
\kern3pt
\nopagebreak
\lipsum[3]
\end{document}
This uses lower level tools and exploits the following facts:
No interline glue is inserted before and after \hrule
A kern in vertical mode is not a feasible page break point if not followed by glue
So the rule with the tiny text cannot go to a new page, nor can there be a page break after it. If you want to allow it, remove the last \nopagebreak
.
Controlling the spaces is easier this way.

\\
does not go into vertical mode;\par
definitely does. whether material after\newline
will stay on the same page depends on (1) whether there's room on the page, (2) the setting of\widowpenalty
, and (3) how many lines remain in the paragraph following the\newline
. there are possibly other considerations as well, but i haven't time to test.\par
before the}
it would be closer to the rule in any case if you do that as it would be set with its own baslineskip not that of normalsize\\
does not go into vertical mode here but it does in some other contexts, notablycenter